A method of manufacturing an optical modulator, e.g., an LN-waveguide modulator, includes forming a Mach-Zehnder interferometer (MZI) type waveguide pattern of titanium (Ti) on an LiNbO3 (LN) wafer. Subsequently, the Ti on the wafer is thermally diffused by an ordinary manufacturing method. Since the LN wafer is a pyroelectric material, the entire wafer is electrically charged by heating the wafer.
The method of manufacturing the modulator has a problem in that the accumulated electrical charge is discharged in the waveguide pattern and results in the waveguide being formed in a damaged state. In a conventional technique, a thin-line dummy pattern is arranged along an input/output waveguide on the input/output side at the time of transfer of a Ti waveguide pattern so as to diffuse the electrical charge and suppress the waveguide damage (see, e.g., WO 1994/010592). In another conventional technique, when Ti is thermally diffused, an ionized gas is used so that electrical discharge is minimized to suppress the waveguide damage (see, e.g., Japanese Laid-Open Patent Publication No. H04-289804). In another disclosed technique, a solid planar pattern is arranged inside an MZI to suppress electrical discharge (see, e.g., Japanese Laid-Open Patent Publication No. 2015-111193).